Km and k cat values were determined for human Hageman factor (factor XII) fragment(s) (HFf) with plasma prokallikrein and six synthetic substrates. A monospecific antiserum to HFf was obtained in sheep. New plant inhibitors of HFf, plasma and urinary kallikreins, factor X, thrombin were detected. SDS-Polyacrylamide gel electrophoresis of the four main electrofocusing forms of human plasma prokallikrein gave a band at M equals 84,000 for each. The pI 9.3 form also had an equally strong band at Mr equals 80,000, while the pI 9.1 and 8.8 forms had progressively weaker bands at this Mr. HFf treatment gave the same pattern. Sheep antiserums to the pI 8.8 prokallikrein and to the kallikrein derived from it were monospecific except for some anti-IgG in the antiserum to prokallikrein. Human plasma kininogens, prepared by immunoaffinity chromatography in the presence of Polybrene and several serine protease inhibitors, gave a DEAE-cellulose chromatographic profile very similar to that found previously for kininogens purified in the presence of Polybrene only.